Method and apparatus for using back EMF to control a motor

ABSTRACT

A circuit to control a motor having phase inputs. A respective control signal for each phase input of the motor is generated such that each phase input is cyclically driven and undriven. Additionally, changes at each phase input relative to a reference potential are measured, and changes at each phase input during time intervals in which noise is expected are ignored: timing information based on the measured changes at the phase inputs is generated, and timing information to carry out the step of generating a respective control signal is used.

TECHNICAL FIELD OF THE INVENTION

This invention relates to the field of electronic motor control and,more specifically, to a method and apparatus for using back EMF tocontrol a motor.

BACKGROUND OF THE INVENTION

Magnetic disk devices, such as hard disk drives supporting randomaccess, utilize a spindle that includes a collection of platters. Theseplatters are covered with a magnetic material for recording information.Each platter contains a series of circular recording tracks containingsectors of information that can be read or written to by electromagneticheads utilizing switchable magnetic fields. The platters of a spindlegenerally rotate at a constant angular speed when memory sectors arebeing read or written.

The rotation of a spindle of platters in a hard disk drive is effectedby a spindle motor. The motor includes a magnetic rotor rotating inresponse to an electrical field created by three sets of electric coils.At any given point in time, only two of the three sets of coils aredriven. As the rotor magnet sweeps past these coils, it generates on theundriven set of coils a back electromotive force (BEMF) signal whichvaries in intensity. The strength of the BEMF signal can be utilized bya hard disk drive controller to provide feedback about the speed andstate of the spindle motor during operation.

In particular, the BEMF signal can be compared to a voltage at a centertap of the motor, which is a common potential tied to each of the threeconnected sets of motor coils, in order to adequately gauge the exactmoment of commutation. This moment of commutation refers to the exacttime when a given electric field produced by a set of coils turns on oroff.

A problem arises when attempting to ensure that the spindle motor of ahard disk drive rotates at a constant speed specified by a hard diskdrive controller. Previous techniques for ensuring that a spindle motorrotates at a designated constant speed include well known controltechniques such as a phase locked loop control system to access andtrack the BEMF strength or phase. However, the phase locked loopdetection technique cannot be easily resynchronized if it falls outsideof a certain range of disparity between the intended and actual phase ofthe BEMF signal. Once the phase falls outside of the set range, thespindle motor must be powered off and/or started again from zero speedin order to resynchronize and accurately track a phase of the BEMFsignal provided by the spindle motor. Additionally, the time required toalter the speed of rotation for the motor is high, due to a largecapacitance in a low pass filter utilized by the phase locked looptechnique. The capacitance in the low pass filter must be maintained ata large magnitude in order to reduce noise that routinely disrupts thecircuit's performance.

The rotational speed of a spindle motor in a hard disk drive may changedue to physical bumping, jarring, vibrations, the interruption of apower supply, an unclear supply line, or a power supply spike, forexample. Thus, it may be seen that a method of synchronizing the speedof a spindle motor with a controller is needed that allows adjustmentafter a significant disruption, and that enables rapid adjustment of therotational speed by a hard disk drive controller to ensure that theoperation of the spindle motor of a drive meets desired performancecriteria.

Another significant source of interference that has the potential tocause disruptions in the constant rotation of a spindle motor is thegeneration of noise by the spindle motor itself. Such noise caninterfere with the ability of BEMF signal strength to accurately reflectthe proper timing of applying inputs to a spindle drive motor. Inparticular, noise is created during the linear operation of a motor justafter points of commutation corresponding to time intervals when a phaseinput to the spindle motor is altered. For a spindle motor operating inpulse width modulation mode, an additional source of noise androtational disruption is found during time intervals when high frequencyswitching of a phase input occurs.

SUMMARY OF THE INVENTION

Accordingly, it may be appreciated that a need has arisen for a methodand apparatus for an improved BEMF detection technique thatsubstantially eliminates or reduces the above-discussed disadvantagesand problems associated with current detection techniques.

In one embodiment of the present invention, an apparatus for controllinga motor having a plurality of phase inputs includes generating arespective control signal for each phase input of the motor such thateach phase input is cyclically driven and undriven, measuring changes ateach phase input relative to a reference potential, ignoring changes ateach phase input during time intervals in which noise is expected,generating timing information based on the measured changes at the phaseinputs, and utilizing the timing information to carry out the step ofgenerating a respective control signal.

BRIEF DESCRIPTION OF THE DRAWINGS

A better understanding of the present invention will be realized fromthe detailed description which follows, taken in conjunction with theaccompanying drawings, in which:

FIG. 1 is a simplified diagram of an apparatus which includes anelectrical motor and a control system that controls the motor using theback EMF (BEMF) of the motor, in accord with the present invention;

FIG. 2 is a schematic diagram of a comparator and BEMF detection devicewhich are components of the control system of FIG. 1;

FIG. 3 is a detailed schematic diagram of the BEMF detection device ofFIG. 2;

FIGS. 4A through 4F are similar diagrammatic views of the motor of FIG.1, in different phases of operation;

FIG. 5 is a timing diagram illustrating the operation of the apparatusof FIG. 1 in a linear operating mode; and

FIG. 6 is a timing diagram illustrating the operation of the apparatusof FIG. 1 in a pulse width modulation operating mode.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, a block diagram is shown of an apparatus 8 whichincludes a control system 10 for a spindle drive motor 20. Controlsystem 10 includes: comparators 22, 24 and 26; back electromotive force(BEMF) detection devices 28, 30 and 32; a combinational logic and statemachine component 34; and a spindle driver device 36. Control system 10is operable to cause motor 20 to rotate at a desired constant speedassuming that spindle drivers are properly controlled. Morespecifically, control system 10 provides voltage input signals to themotor 20 that cause rotation at a constant predesignated angularvelocity, detects BEMF signals generated by motor 20 in response to suchrotation, filters the BEMF signals to discount the effects of noise andinterference, and then utilizes the filtered signals to produce thecorrect commutation signals to ensure that the predesignated velocity ismaintained.

As shown in FIG. 1, motor 20 has three phase inputs that are coupled tooutputs of spindle driver device 36 and to inputs of comparators 22, 24and 26. Comparators 22, 24 and 26 have outputs coupled to respectiveinputs of BEMF detection devices 28, 30 and 32. Each of the BEMFdetection devices 28, 30 and 32 has an output coupled to a respectiveinput of combinational logic and state machine component 34. Finally,component 34 has an output coupled to an input of spindle driver device36. Other components not explicitly shown may include a reference clock,a speed control input device, and a power supply.

Spindle drive motor 20 is well known in the hard disk drive industry,and includes a magnetic rotor 40 having two poles: a north pole 42 and asouth pole 44. Rotor 40 is surrounded by three electric coils 46, 48 and50, each of which is implemented with a pair of coils disposed ondiametrically opposite sides of the rotor. Various combinations ofvoltage signals are applied to coil pairs 46, 48 and 50 by spindledriver device 36 to generate current in the coils. As is generallyknown, passing current through coil pairs 46, 48 and 50 will create anelectromagnetic field operable to cause magnetic rotor 40 to rotate,thereby operating the spindle motor of the hard disk drive. Morespecifically, applying a specified series of voltage signals to eachpair of coils in synchronism with the orientation of rotor 40 causescurrent to flow through the coils in directions causing rotor 40 tomaintain a near constant velocity of angular rotation. For example, theorientation of rotor 40 in FIG. 1 is such that the north pole 42 ofrotor 40 is pointing between the north pole of coil pair 46 and undrivencoil 48, 30° clockwise from a parallel alignment with coil pair 46. Analignment such as this is hereafter referred to as a point ofcommutation. The voltages applied in such an example are generated so asto cause the rotor to continue to rotate past coil pair 46 in aclockwise direction. The exact voltage inputs applied are describedbelow in relation to FIGS. 4-6.

Coil pairs 46, 48 and 50 are connected at a common potential known inthe industry as a center tap 52. Center tap 52 is generally at apotential which is roughly half the magnitude of the power supply formotor 20. For example, a spindle motor operating off of a 12 volt DCpower supply would display a center tap voltage at approximately 6volts.

During the rotation of rotor 40, at any given time, two of the coilpairs will be "driven" and have current passing through them in responseto the voltage inputs applied by spindle driver device 36 to the coils.The remaining coil pair is designated as being undriven, and has nocurrent generated in response to the voltage signals applied. However,as magnetic rotor 40 rotates, an electromotive force is created, causingflux be generated at the undriven coil pair. A resulting potential, thepreviously identified BEMF signal, can then be measured at the input tothat specific undriven coil pair. The strength of this BEMF signaldepends on the orientation of the magnetic rotor in relation to coils46, 48 and 50. As the north pole of the magnetic rotor approachesalignment with the undriven coil pair, the magnitude of the BEMF signalwill approach, cross, and begin to move away from the center tapvoltage. This process is addressed in further detail and in relation toother control signals in the description accompanying FIGS. 4 through 6.

The BEMF signal, or more precisely the voltage measured at the input toeach coil pair 46, 48 and 50 when that pair is undriven, is measuredover time by comparators 22, 24 and 26, by comparing the signal to thevoltage at center tap 52, which is common to all pairs. Comparators 22,24 and 26 each generate a digital output known as a comparator outputsignal, labeled C_(a), C_(b) and C_(c) respectively. The digital outputessentially changes values from a low to a high value or from a high toa low value each time the BEMF signal crosses the center tap voltage 52.Thus, a rising or falling edge of a comparator output signal correspondsto the voltage measured at the input of a corresponding coil paircrossing center tap voltage 52 as it increases or decreases inmagnitude. The operation of comparators 22, 24 and 26, while generallyknown in the industry, will be described in more detail later, inassociation with FIG. 2.

Comparator output signals C_(a), C_(b) and C_(c) are then utilized asinput signals for BEMF detection devices 28, 30 and 32. BEMF detectiondevices 28, 30 and 32 take the center tap crossing points indicated bythese comparator output signals, and retain or reject their validitybased on the predicted presence of noise and/or interference. Morespecifically, devices 28, 30 and 32 filter out false center tapcrossings caused by the switching of the voltage inputs to spindle drivemotor 20. This switching includes both switching of input signals uponcommutation of the rotor with a coil pair and the pulsing of inputsassociated with the operation of motor 20 in pulse width modulationmode. Other sources of noise or interference that occur outsidepredicted intervals of true center tap crossings may be rejected usingthe BEMF detection devices described herein. BEMF detection devices 28,30 and 32 then generate BEMF detection signals B_(a), B_(b) and B_(c)respectively that are similar to the comparator output signals in thatthey reflect a rising or falling edge for each true center tapcrossings. Any false center tap crossings caused by interference uponcommutation or upon input pulsing will have been eliminated from thesignal by devices 28, 30 and 32. The operation of devices 28, 30 and 32will be addressed in more detail below in the description of FIG. 3.

The BEMF detection signals are then translated by combinational logicand state machine component 34, which may include combinational logiccurrently used in the hard disk drive industry to detect points ofcommutation based on center tap crossings. However, whereas thecombinational logic and state machine components of prior spindle drivemotor controllers utilized comparator output signals to identify centertap crossings, the present invention utilizes the filtered true centertap crossings identified by BEMF detection devices 28, 30 and 32, toachieve greater accuracy and efficiency. Component 34 essentially delaysa determined period of time after each center tap voltage crossing, andthen generates a commutation output to the spindle driver deviceindicating a predicted point of commutation. In calculating that periodof time between crossing and commutation, a reference clock is utilizedalong with data from the previous state of a state machine withincomponent 34. The state machine includes further data regarding theprevious crossing point and point of commutation. Component 34 mayutilize additional external controllers to determine the proper periodof delay for a particular state.

Spindle driver device 36 utilizes the commutation output signals ofcomponent 34 to apply controlled voltage inputs V_(a), V_(b) and V_(c)to coil pairs 46, 48 and 50 respectively. As has already been explained,in order to keep the spindle motor rotating at a constant speed, a harddisk drive controller must be able to detect any legitimate crossings ofthe center tap voltage value by BEMF signals at inputs to each coil pairof the motor. The spindle driver device then utilizes these cross overpoints to engage switching mechanisms, whereby the fields created bycoils 18, 20 and 22 can be either turned on or off to ensure constantrotation by the magnetic rotor.

For purposes of brevity and clarity, the description accompanying FIGS.2 and 3 will be in regards to a single coil pair 46 of spindle drivemotor 20 shown in FIG. 1, and its associated comparator 22 and BEMFdetection device 28.

Traditionally, as already mentioned, detecting a cross over point isaccomplished by the use of comparator 22 as shown in FIG. 2. Comparator22 includes an amplifier having as inputs the two analog signals fromthe center tap and the BEMF output, and producing at an output a digitalcomparator output signal C_(a) indicating whether or not cross over hasoccurred. Generally, comparator 22 compares the center tap voltage tothe BEMF signal, causing the comparator output signal C_(a) to go fromlow to high or from high to low whenever a cross over has occurred.

However, comparator 22 does not have the capability to filter out largesources of noise created during the operation of spindle motor 20. Onesource of large electric noise occurs immediately after drivers withinspindle driver device 36 of FIG. 1 switch on or off voltage inputs toany of electric coil pairs 46, 48 or 50. Noise may be generated as anelectrical signal due to inductor kickback after a specific coil hasbeen turned off, for example. This is described in more detail in thedescription accompanying FIGS. 4-6.

Another source of large noise can be attributed to switching that takesplace when spindle motor 20 operates in a pulse width modulation mode.Essentially, during pulse width modulation mode, electric coil pairs 46,48 and 50 of spindle motor 20, instead of potentially being switched onduring an entire period between commutations, will instead be rapidlyswitched on and off during the same period. This pulse signal closelyapproximates the signal created by the coil if it were operated in aconstant "linear" mode, by averaging over the entire time period betweencommutations the value of the pulse strength during all of the pulsesthat take place in one period between commutations. Thus, pulse widthmodulation mode can closely approximate linear operation while wastingless energy on the driver devices themselves. The switching associatedwith each of these pulses can again be the source of a large noisesignal. Neither of these sources of noise are small enough to becompensated for by a simple comparative circuit like that of comparator22.

Fortunately, the moments of commutation and the switching times of thePWM mode occur at controlled times. Timing information about thesemoments can be used to mask out the comparator output when an output islikely to be false, by utilizing BEMF detection device 28, which is alsoshown in FIG. 2.

BEMF detection device 28 utilizes the comparator output signal to detecttrue crossing of the center tap voltage based on known sources of noise.BEMF detection device 28 generates a BEMF detection signal B_(a),indicating when true crossings of the center tap voltage have occurred.

BEMF detection device 28 utilizes four inputs. The first input is thecomparator output signal C_(a) generated by comparator 22, as describedabove. A second input is an active low PWM signal generated by spindledriver device 36, indicating that a switching moment used to generate apulse is about to occur. This PWM signal essentially indicates that acomparator output thereafter received by BEMF detection device 28 may becorrupted by PWM switching noise. A third input to the device 28 is anactive low commutation signal that is generated by combinational logicand state machine component 34 of FIG. 1 when switching associated withcommutation is predicted to occur. This commutation signal becomesactive at the point of commutation, and remains active for approximately15 degrees of the phase of the magnetic rotor, for example. The fourthinput to BEMF detection device 28 is a mode indicator that provides BEMFdetection device 28 with a current operating mode of the spindle drivemotor. The mode indicator indicates to BEMF detection device 28 whetherspindle drive motor 20 is operating in the linear mode or in the pulsewidth modulation mode. BEMF detection device 28 then utilizes thatinformation to decide whether or not the PWM signal should be utilizedin determining true crossings of the center tap voltage by the BEMFsignal. The four inputs are utilized by BEMF detection device 28 togenerate BEMF detection signal B_(a) for use by the combinational logicand state machine component 34 of FIG. 1.

Referring now to FIG. 3, a more detailed circuit diagram of BEMFdetection device 28 is shown. BEMF detection device 28 has the fourinputs as previously described in the discussion of FIG. 2. BEMFdetection device 28 includes two stages, in particular a pulse stage 60and a commutation stage 80.

Pulse stage 60 compensates for magnetic switching associated with pulsewidth modulation mode operation of spindle drive motor 20, by maskingthe comparator output signal during times associated with probable noisefrom such switching. FIG. 3 shows that pulse stage 60 includes selectiondevices 62, 64 and 66, register 68, invertors 70, 72 and 74, and delayelement 76.

Selection device 62 of pulse stage 60 utilizes two inputs, which are thecomparator output signal and the mode indicator. Device 62 may be ademultiplexer, for example, capable of having its input signal presentedat either of two output nodes, dependent on its selection input "sel"that is connected to the mode indicator. If the mode indicator is low,signifying the linear operation mode of the spindle drive motor,selection device 62 will select the Y1 output line on which to send thecomparator output signal to selection device 66 as the first input at A.If the mode indicator is high, signifying the pulse width modulationmode of the spindle drive motor 20, device 62 will select the Y2 outputline on which to send comparator output signal to register 68 andselection device 64. Specifically, the comparator output signal connectsto register 68 as an input at D.

Register 68 may be any register or similar buffering device, such as alatch, for example. Register 68 has two input signals and one outputsignal. The first input is input D that, as already mentioned, providesan input node for the comparator output signal from selection device 62.The second input to register 68 is a clock signal input C, by whichregister 68 adjusts its operation. Essentially, register 68 latches inthe value of the comparator output signal from input D at output Q eachtime there is a rising edge of the clock signal at input C.

The clock signal at C is a signal derived from the PWM signal utilizedas an input to detection device 28. Specifically, the active low PWMsignal passes through an inverter 70 before reaching register 68 atinput C. As previously indicated, PWM signal goes low each time aswitching moment used to generate a pulse is about to occur. This lowsignal is then inverted by inverter 70 and supplied to register 74 as aclock signal. As such, the beginning of a pulse is actually seen byregister 68 as the rising edge of the clock signal at C. Any time apulse associated with spindle drive motor 20 is detected, the clocksignal at C will switch from low to high, thus displaying a leading edgeutilized by register 68 to latch in the comparator output signal inputat D to the output at Q.

The output held at Q essentially represents the last reliable comparatoroutput signal generated by comparator 22 that is not corrupted by noisegenerated during pulse width modulation. The output at Q is in turncoupled to selection device 64 as its second input at B. Detectiondevice 64 has two inputs, namely the unmodified BEMF comparator outputsignal relayed from selection device 62 at input A, and the last latchedsignal from register 68 present at output Q at input B.

One of the two inputs to selection device 64 is selected by a controlsignal present at the selection input "sel", to be presented at output Yof selection device 64. Selection input to selection device 64 is thePWM input signal after being inverted and delayed. More specifically, asseen at the selection input, the PWM signal input to the BEMF detectioncircuit is first inverted by inverter 70, as previously indicated, andthen passes through additional invertors 72 and 74 and their associateddelay element 26. The PWM signal, after being inverted by inverter 74,is supplied to selection device 64 as its selection input.

The delay introduced by delay element 76 may be a period of 6nanoseconds, for example. The delay element 76 is a transistor presentbetween the output of inverter 72 and the input of inverter 74 as shownin FIG. 3. This single transistor has its source, drain and body or bulktied to ground, while the gate of the transistor is tied to the nodeexisting between invertors 72 and 74. In such a configuration, thetransistor 76 functions as a capacitative element that must be chargedbefore the signal at the output of inverter 72 enters as an input toinverter 74.

The delay introduced by delay element 76 ensures a stable output fromregister 68 at Q, thereby ensuring a smooth transition at output Y ofselection device 64. The purpose of this delay is to ensure that theinput at D is given time to latch in at output Q of register 68. Delayelement 76 specifically prevents the selection input of selection device64 from switching the value at output Y from the value at input A to thevalue at input B prior to the time that Q is set to a different valuefollowing a rising clock edge. Without the presence of delay element 76,selection device 64 could switch output Y to the input at B, and thatinput could subsequently change values as the input at D is latched intoQ. Such an instability at output Y of selection device 64 could lead toBEMF detection device 28 inaccuracies in masking noise associated withpulse width modulation.

Thus, selection device 64 acts to mirror the comparator output signalduring periods of time where no noise associated with pulse widthmodulation is expected, and acts to reflect the latched value of thecomparator output signal at its latest reliable point when noise isexpected in association with pulse width modulation.

Coupling the pulse stage 60 to commutation stage 80 is selection device66. As already mentioned, selection device 66 has as its first input amirrored comparator output signal routed directly from selection device62, that avoids all intermediate devices of pulse stage 60. Theselection input "sel" for selection device 66 is the mode indicatorpreviously discussed, that designates whether spindle drive motor 20 isoperating in linear or pulse width modulation mode. When spindle drivemotor 20 is operating in linear mode, selection device 66 simply passesthe unmodified input A to output Y, effectively bypassing the entirepulse width modulation stage. However, if the mode indicator indicatesthat motor 20 is operating in pulse width modulation mode, selectiondevice 66 passes the comparator output signal at input B to output Y toserve as an input to pulse stage 60. In this way, pulse stage 60 isutilized only when spindle drive motor 20 is operating in pulse widthmodulation mode, thereby filtering out the comparator output signalduring predicted periods of probable pulse noise interference.

Commutation stage 80 has as inputs the comparator output signal seen atoutput Y of the PWM stage, and the active low commutation signalpreviously discussed. Commutation stage 80 includes a register 90, andselection device 92, as well as a similar inverter and delay elementconfiguration to that seen in pulse stage 60. The inverter and delayelement configuration includes invertors 94, 96 and 100 and delayelement 98.

The comparator output signal from pulse stage 60 is supplied to both theselection device 92 and register 90. Specifically, the comparator outputsignal is the first input A of selection device 92. The comparatoroutput signal from pulse stage 60 is also supplied to register 90 atinput D.

Register 90 is similar to register 68. Like register 68, register 90 hasa clock input at C. The clock input is the active low commutationsignal, after it passes through an inverter 94. Thus, similar to theoperation of register 68, a falling edge of the commutation signal isreflected with a rising edge of the clock signal utilized by register90. Specifically, as register 90 sees a rising edge of the clock signal,the comparator output signal present at input D is latched in at outputQ of register 90. The commutation signal has a falling edge at a pointof commutation of the magnetic rotor 40 and a coil of the spindle drivemotor 20. The commutation signal therefore acts to preserve a lastreliable comparator output signal at output Q of register 90 immediatelyprior to periods of time associated with noise or interference caused bycommutation. It should be noted that the commutation signal is reset toa high value, and therefore inactive status, by combinational logic andstate machine device 34 after a specific phase delay following rotorcommutation. For example, as described later, the period of time beforethe reset may correspond to a rotation of 15° of the magnetic rotorfollowing commutation.

The latched comparator output signal at output Q of register 90 is thesecond input B of selection device 92. The selection input "sel" ofselection device 92 is the commutation signal seen by selection device92 after passing through the inverter delay configuration previouslymentioned. More specifically, after the commutation signal has passedthrough inverter 94, the commutation signal then passes through inverter96, and is then delayed by delay element 98. Delay element 98 may,similar to element 96, be a single transistor having its source, drainand body or bulk tied to ground, and its gate tied to the output ofinverter 96 and the input of inverter 100. Such a transistor can beselected based on its size and properties to achieve a desired delay.The transistor in FIG. 3, generates a delay of six nanoseconds. As inthe previous stage, this delay essentially allows time for the BEMFcomparator input at D to latch at output Q of register 90 upon seeing achange in the commutation signal at the clock input.

After experiencing this delay, the commutation signal passes through afinal inverter 100 to be seen at the selection input of selection device92. This selection input will cause output Y to carry the comparatoroutput signal from input A during times when the commutation signal hasnot indicated that a period of commutation noise or interference isabout to occur. However, when the commutation signal has identified sucha period, selection device 92 will instead supply to its output thelatched value of the comparator output signal from output Q of register90. The output of selection device 92 is hereafter referred to as a BEMFdetection signal and identified in FIG. 1 as B_(a). Thus, selectiondevice 92 outputs the comparator output signal from pulse stage 60,except for periods of time immediately upon and for a short time aftercommutation occurs.

Commutation stage 80 therefore filters out noise and interference thatis associated with commutation and that may corrupt an accurate BEMFdetection signal being utilized by spindle driver device 36 as describedin association with FIG. 1.

FIGS. 4A to 4F are diagrams showing the phases of operation of spindledrive motor 20 at 60° increments. More specifically, FIG. 4 shows theapplication of voltage input signals to coil pairs 46, 48 and 50 duringthe angular movement of magnetic rotor 40. For example, FIG. 4A showsthat the voltage input signal applied to coil pair 46 is a shortcircuited connection to ground, designated at V_(a) by S. In the sameFIGURE, the voltage input supplied to coil pair 48 is an open circuit.The voltage input signal applied to coil pair 50 in FIG. 4A is apositive voltage of 12 volts, for example. With such an application ofsignals, coil pairs 46 and 50 represent coil pairs driven by theapplication of the positive input voltage signal at V_(c). The potentialacross undriven coil pair 48 can then be measured by comparator 24 todetect the presence of a back electromotive force created by therotation of magnetic rotor 40 as previously described. FIG. 4 will bedescribed in greater detail in association with the timing diagramsfound in FIGS. 5 and 6.

FIG. 5 is a timing diagram summarizing the relationship between thesignals identified in FIG. 1 during the linear operation of a spindledrive motor. The timing diagram in effect derives, filters, or otherwisecontrols the signal detecting back electromotive force of motor 20.

Specifically FIG. 5 first illustrates the values of voltage inputsV_(a), V_(b), and V_(c) as applied to coil pairs 46, 48, and 50respectively. The timing diagram shows the three signals as they varyrelative to the angular position of magnetic rotor 40 of the spindledrive motor 20. More specifically, the timing diagram shows thosesignals at a continuous spectrum across the angle of rotation andprovides references every 60° of that rotation.

The timing diagram begins with the magnetic rotor in a slightly righttilted position as shown in FIG. 4A, corresponding to a 0° angle ofrotation. At that angle, the spindle motor driver 36 shown in FIG. 1 isapplying a positive voltage to coil pair 50 while effectively shortcircuiting the input signal to coil pair 46 and causing an open circuitat the input to coil pair 48. Such a configuration of applied inputsignals causes current to flow from the applied voltage source at V_(c)through the pair of coils 50. After passing through coil pair 50 thecurrent sees an open circuit at coil 48 and a direct connection toground through the short circuited path including coil pair 46. Thecurrent therefore follows the current path through the pair of coils 46,completing a closed circuit. Current thus flows through coils 46 and 50in the direction indicated by the arrows in FIG. 4A. The electromotiveforce generated across coils 46 and 50 causes the angular rotation ofmagnetic rotor 40 in a clockwise direction. As magnetic rotor 40rotates, the back electromotive force of motor 20 can be detected atcoil 48 and measured as an output relative to the voltage at center tap52 by comparator 24 associated with coil 48.

As the rotor progresses from 0° to 60°, a positive voltage is applied tothe input for coil 48. Initially, as shown in FIG. 5, a voltage spike isseen at V_(b) as that input signal sees the initially open circuit as alarge resistor, corresponding to an equally high voltage. Once the spikehas passed, V_(b) relatively gradually approaches a full strengthpositive voltage at 60°. As the angle of rotation of magnetic rotor 40reaches 60°, the voltage input for coil 50 is cut off, creating an opencircuit. Initially, a spike drastically decreasing the input signalvoltage for coil pair 50 is experienced due to inductor kick-back.Following kick-back, the voltage at 50 gradually decreases as the rotor40 continues to rotate from 60° to 120°, eventually reaching ground or alow voltage at 120°. Thus, at 60° coil 48 is driven by a positive inputvoltage, coil 46 effectively sees a short circuit at its input, and coil50 is effectively an open circuit. Thus similar to current flow at 0°,current at 60° originates at the positive input signal introduced at theinput to the coils 48 and follows the current path including coils 46directly to ground. A back electromotive force is seen at the input tocoil C, and is measured relative to the center tap voltage by comparatorcircuit 26 associated with coil pair 50 as shown in FIG. 1.

The remaining rotation of the rotor from 120° to 360°, or 0°, canreadily be understood by looking at FIGS. 4 and 5. It should be notedthat a center tap crossing of a BEMF signal occurs during the undriven,or open circuited, phases of each coil pair.

FIG. 5 also illustrates the noise associated with commutation shortlyafter each 60° increment of angular rotation. Specifically, shortlyafter 0°, corresponding to magnetic rotor 40 being approximately 30°clockwise relative to coil pair 46, the BEMF signal measured at theinput to coil pair 48 may experience noise or disruption associated withswitching the input signals to each of the three coil pairs. This mayresult in the generation of a false center tap crossing indication whencomparator 24 compares the potential at coil pair 48 to the center tapvoltage such as the crossing shown after 0° by C_(b). Similar falsecrossings are illustrated for coil pair 46 at comparator 22 shortlyafter 120° and 300° as shown by signal C_(a) in 22 in FIG. 5, and forcoil pair 50 at comparator 26 shortly after 60° and 240° as shown byC_(c) in FIG. 5. An additional false center tap crossing is illustratedby C_(b) shortly after 180° for coil pair 48.

Similarly, FIG. 5 shows how the implementation of a commutation signalmay be utilized by the BEMF detection device illustrated in FIG. 1 toblock or screen out these false center tap crossings. More specifically,as shown in FIG. 5, the angular period of rotation between each fallingedge of the commutation signal and the subsequent rising edge of thecommutation signal identifies a period of angular rotation when a BEMFsignal associated with a specific coil may indicate that a false centertap crossing has occurred. Essentially, in the embodiment described byFIG. 5, the falling edge of the commutation signal correspondsapproximately with each 60° increment of the magnetic rotor's rotation,with the rising edge occurring approximately 15° after the initialfalling edge. Effectively, as rotor 40 lines up to the mid point witheach adjacent set of coils of the spindle drive motor, the commutationsignal is set until the rotor passes those coils by an angular margin of15°.

Additionally, FIG. 5 shows the presence of the BEMF detection signals asoutput from each BEMF detection device as shown in FIG. 1 as B_(a),B_(b), and B _(c) relative to the previously described control signals.Specifically, B_(a), B_(b) and B_(c) of FIG. 5 display rising andfalling edges of the BEMF detection devices' output after filtering outof all center tap crossings associated with commutation noise and/orinterference occurring during the linear operation of the spindle drivemotor. For example, signal C_(a) shows the presence of six rising andfalling edges possibly corresponding to center tap crossings. After thefilter effect imposed by BEMF detection device 28, the number of risingand falling edges shown by signal B_(a) accurately reflects two truecenter tap crossings of the BEMF signal generated at coil pair 48.

FIG. 6, similar to FIG. 5, shows the filtering of noise and interferenceassociated with pulse signals experienced during the operation ofspindle drive motor 20 in pulse with modulation mode. FIG. 6specifically shows the same noise and interference associated withcommutation as previously described relative to FIG. 5. In addition,FIG. 6 shows additional noise or interference caused by the rapidswitching, or pulsing, of the voltage input signals to each set of coilsas the spindle drive motor operates during pulse width modulation.Essentially, whereas during linear operation a voltage input signal to aset of coils may be consistently held to a high voltage for a period ofapproximately 120°, during pulse width modulation operation that voltageinput signal would instead be rapidly pulsed, or switched on and off, ata frequency designed to approximate a constantly held voltage. Suchpulse width modulation may maintain the same angular velocity ofrotation, while requiring potentially less overall power as previouslydescribed.

Obviously, as shown in FIG. 6, during the period of time when a voltageinput signal is pulsed there will be center tap crossings of the centertap voltage by the BEMF signals. These crossings are caused by eachpulsed input signal, and are crossings that do not correspond to a BEMFsignal actually crossing the center tap voltage. FIG. 6 thus shows thepresence of a PWM signal similar to the commutation signal, whicheffectively blocks the reading of a BEMF voltage during periods of timethat pulse switching is expected. This blocking is accomplished by theBEMF detection device previously described. Once such interference hasbeen eliminated, as shown in FIG. 6, B_(a), B_(b), and B_(c) thenaccurately reflect the true number of center tap crossings.

The present invention provides various technical advantages over currentdetection techniques. For example, one technical advantage is that theimproved BEMF detection technique of the current invention allowsadjustment and resynchronization of spindle motor speed after asignificant disruption or disparity is detected. Another technicaladvantage is that the present invention allows the adjustment andresynchronization of the spindle motor rotation speed without thesignificant delay associated with current detection techniques. Anothertechnical advantage is that hard disk drive technology utilizing thecurrent invention will not need to power off a hard disk drive when asignificant disparity between the actual spindle motor rotation speedand intended speed is detected. Additionally, the invention provides fora system and method of maintaining the rotation speed of a spindle motorthat is not significantly vulnerable to electrical sources of noise orphysical interference.

It will be understood by one possessing ordinary skill in the art thatthe specific selection of device components and connections describedthroughout this specification does not in any way limit the scope ofthis invention. For example, other configurations or components could beutilized which effectively block the detection of false center tapcrossings by tracking the occurrence of interference associated withcommutation and pulse width modulation without departing from the scopeof the present invention. In addition, other sources of noise and/orelectrical interference may be similarly eliminated through theutilization of the techniques disclosed herein without departing fromthe scope of this invention. Thus, although one embodiment has beendescribed in detail, it should be understood that various and numerouschanges, substitutions, and alterations can be made therein withoutdeparting from the scope of the present invention. Other examples ofchanges, substitutions, and alterations are readily ascertainable by oneskilled in the art and could be made without departing from the spiritand scope of the present invention as defined by the following claims.

What is claimed is:
 1. An apparatus for controlling a motor having aplurality of phase inputs, comprising:a drive circuit that has aplurality of phase outputs and that is operative to control each of saidphase outputs according to a predetermined sequence of driven andundriven states; and a control circuit operative to provide timinginformation to said drive circuit in response to a signal at each phaseoutput, said control circuit being operative to be nonresponsive to thesignal at each said phase output during predetermined time intervals ofthe predetermined sequence.
 2. The apparatus of claim 1, wherein eachsaid phase output is cyclically driven and undriven during respectivesuccessive first and second time intervals, the second time interval foreach said phase output being offset in time from the second timeinterval for each of the other phase outputs.
 3. The apparatus of claim1, wherein each said phase output is cyclically driven and undrivenduring respective successive first and second time intervals, the secondtime interval for each said phase output being offset in time from thesecond time interval for each of the other phase outputs, said controlcircuit being nonresponsive to each of said phase output of said drivecircuit during a predetermined portion of the first time interval ofthat particular phase output.
 4. The apparatus of claim 1, wherein eachsaid phase output is cyclically driven and undriven during respectivesuccessive first and second time intervals, the second time interval foreach said phase output being offset in time from the second timeinterval for each of the other phase outputs, said control circuitignoring each said phase output of said drive circuit during apredetermined portion of the second time interval of that particularphase output.
 5. The apparatus of claim 1, wherein said control circuitignores each said phase output during predetermined time intervals whennoise is expected at that particular phase output.
 6. The apparatus ofclaim 1, wherein said control circuit comprises a plurality ofcomparators, each comparator coupled to one of the phase outputs, eachcomparator being operable to compare the signal at the phase output to areference potential, each comparator being further operable to generatea respective comparator output signal, each comparator being furtheroperable to modify the respective comparator output signal in responseto comparing the signal at the phase output to the reference potential.7. The apparatus of claim 1, wherein said control circuit comprises aplurality of comparators, each comparator coupled to one of the phaseoutputs, each comparator being operable to compare the signal at thephase output to a reference potential, each comparator being furtheroperable to generate a respective comparator output signal, eachcomparator being further operable to modify the respective comparatoroutput signal in response to comparing the signal at the phase output tothe reference potential; andwherein said control circuit furthercomprises a detection circuit that is operative to latch the comparatoroutput signal of one of the comparators during time intervals wheninterference is expected that is associated with the phase outputcoupled to that comparator.
 8. The apparatus of claim 1, wherein saidcontrol circuit comprises a plurality of comparators, each comparatorcoupled to one of the phase outputs, each comparator being operable tocompare the signal at the phase output to a reference potential, eachcomparator being further operable to generate a respective comparatoroutput signal, each comparator being further operable to modify therespective comparator output signal in response to comparing the signalat the phase output to the reference potential; andwherein said controlcircuit further comprises a detection circuit that is operative togenerate a detection signal in response to each of the comparator outputsignals, each detection signal ignoring changes to the comparator outputsignal that the detection signal responds to during time intervals whennoise is expected, the timing information generated by the controlcircuit in response to the detection signals.
 9. A method of controllinga motor having a plurality of phase inputs, comprising:generating arespective control signal for each phase input of the motor such thateach phase input is cyclically driven and undriven; measuring changes ateach phase input relative to a reference potential; being nonresponsiveto changes at each phase input during time intervals in which noise isexpected; generating timing information based on the measaured changesat the phase inputs; and utilizing the timing information to carry outsaid step of generating a respective control signal.
 10. The method ofclaim 9, wherein said step of measuring changes at each phase inputincludes modifying a comparator output signal each time the potential ata phase input equals the reference potential.
 11. The method of claim 9,wherein said step of not responding to changes at each phase inputincludes identifying predetermined time intervals where noise associatedwith the switching of phase inputs is expected.
 12. The method of claim9, wherein said step of measuring changes at each phase input includesmodifying a comparator output signal each time the potential at a phaseinput equals the reference potential; andwherein said step of notresponding to changes at each phase input includes ignoringmodifications to the comparator output signal during time intervals whennoise is expected.
 13. The method of claim 9, wherein said step ofmeasuring changes at each phase output includes modifying a comparatoroutput signal each time the potential at a phase input equals thereference potential; andwherein said step of not responding to changesat each phase input includes latching the comparator output signalduring time intervals in which change to the comparator output signal isexpected as a result of noise.
 14. The method of claim 9, wherein saidstep of not responding to changes at each phase input includesidentifying time intervals in which noise is expected due to commutationof the phase inputs.
 15. The method of claim 9, wherein said step of notresponding to changes at each phase input includes identifying timeintervals in which noise is expected due to pulse width modulation ofthe phase inputs.
 16. A motor control system comprising:a spindle drivemotor having a plurality of coils; a driver coupled to said spindledrive motor, said driver being operable to apply phase input signals tosaid coils of said spindle drive motor, the phase input signals beingoperable to cause each said coil to be cyclically in driven and undrivenstates; and a control circuit coupled to said spindle drive motor andsaid driver, said control circuit being operable to measure a change inelectric potential at each said coil relative to a reference potential,said control circuit being further operable to not respond to the changein electric potential during predetermined time intervals in which noiseis expected, said control circuit being operative to send timinginformation to the driver based on the measured changes in electricpotential.
 17. The motor control system of claim 16, wherein saidcontrol circuit comprises a comparator that is operative to modify acomparator output signal each time the electric potential equals thecommon potential.
 18. The motor control system of claim 16, wherein saidcontrol circuit comprises a logic component that is operative togenerate an interference indicator during predetermined time intervalswhere noise associated with switching of the phase inputs is predicted.19. The motor control system of claim 16, wherein said control circuitcomprises a comparator that is operative to modify a comparator outputsignal each time the electric potential equals the commonpotential;wherein said control circuit further comprises a logiccomponent that is operative to generate an interference indicator duringpredetermined time intervals where noise associated with switching ofthe phase inputs is predicted; and wherein said control circuit furthercomprises a detection circuit that is operative to generate a detectionsignal in response to the comparator output signal, the detectioncircuit being further operable to ignore modifications to the comparatoroutput signal in response to the interference indicator, the timinginformation being generated by the logic component in response to thedetection signal.
 20. The motor control system of claim 16, wherein thetiming information is utilized by said driver to control the applicationof the phase inputs.